If we're going to implement this, it'd make sense to have two percentages: an upper limit (set by default to 80%) and a lower limit (set by default to 50%). You get a notification when you hit 80% connected to AC, and another notification when you hit 50% on battery.

We could name it "battery health" mode, or something like that. What do you think?

I too would like to see this feature as this is the reason why I am using the app. Perhaps the easiest way to implement something like this is to have the notifications be show regardless if the battery is charging or discharging. Currently they only seem to show if the battery is discharging, except when the battery is full.